1. Field of the Invention
The present invention generally relates to an optical filter, and more particularly, to an optical filter and an optical amplifier for use in optical communication and optical measurements, etc.
2. Description of the Related Art
Generally, in the optical communication as referred to above, optical amplifiers which amplify light as it is are employed. Through employment of the optical amplifier as a repeater or relay unit, an economical system may be constructed, since it is not required to convert light into an electrical signal. Particularly, for transmission of wavelength division multiplexed signal light, it is possible to transmit signals in large capacities for servicing, since the wavelength division multiplexed light may be amplified at one time by the optical amplifier. For such an optical amplifier, erbium-doped optical fibers have been employed. The erbium-doped optical fiber amplifier as referred to above has such advantages that it has a high amplification factor, with a superior noise characteristic (as described in Microoptics news Vol. 8, No. 1, page 23, 1990).
However, it is known that, in the optical amplifier as described above, the amplification factor thereof varies with respect to the wavelength of its incident signal light, and such wavelength dependency is shown by a dotted line in FIG. 4. Therefore, when the wavelength division multiplexed light is collectively amplified for transmission by connecting the optical amplifiers in many stages, intensities of the signal light for respective wavelengths markedly differ at the receiving point. Thus, depending on channels, received signal power level may vary, with generation of cross talk, thus resulting in a deterioration in the sensitivity for the reception. Accordingly, in order to transmit the wavelength division multiplexed signal in the equal power level, an equalizer or filter is required for smoothing a wavelength depending characteristic. As the optical filters currently used, those employing dielectric multi-layered films are widely utilized. Since the optical characteristics of such an optical filter are determined by the construction of the dielectric multi-layered film, it is difficult to obtain a sharp characteristic or a specifically desired passing characteristic. Moreover, part of the optical fiber amplifier is held between a flat plate and a refraction grating, thereby constituting the filter (IEEE PHOTONICS TECHNOLOGY LETTERS VOL. 3 NO.2 February 1991 P. 118-120).
However, with the conventional optical filter as described above, not any desired wavelength characteristic can be obtained. Furthermore, in the construction to be held between two plates, the size of the entire optical filter becomes large at 40 cm, and in the optical filter of this type, the filter characteristic is unstable, since the filter characteristics are adjusted by the pressure of the two plates. Thus, there have been various problems to be solved in the known optical filter.